Can't I set a variable at property url mx:HTTPService?

I build an application that need httpservice feature. This application will be deployed at some computer. So, I don't have an idea to set variable at url property.

  <mx:HTTPService id="personRequest" url="http://118.97.239.36/dokeos/main/webservices/send_sip.php"
     useProxy="false" method="GET" resultFormat="text" result="personJSON(event)">
    <mx:request xmlns="">
      <getPerson>"true"</getPerson>
    </mx:request>
  </mx:HTTPService>

I already try with block开发者_C百科 {}, but no use.

Can't I set a variable at property url mx:HTTPService?

I personally prefer using actionscript because you can then use the same service over and over but sending different data:

personRequest.send({getPerson:true});